Discussion of casing logically begins with the mold. The mold contains cavity whose geometry determines the shape of the case part . The actual size and shape of the cavity must be slightly oversized to allow for shrinkage that occurs in the metal during solidification and the cooling. Different metals undergo different amounts of shrinkage ,so the mold cavity must be designed for the particular metal to be cast if dimensional accuracy is critical. Molds are made of variety of materials, including sand , plaster , ceramic ,and metal The various casting processes are often classified according to these different types of molds.